The present invention relates to a voltage sensor for measuring voltage and is particularly suitable for measuring high voltages such as on high-voltage power transmission lines.
The list of known high-voltage voltage sensors includes inductive transformers, capacitive dividers or capacitive voltage transformers, and transducers using bulk-optic electric field sensors. The first two sensors suffer from bandwidth restrictions, expensive failure, extensive maintenance, heavy weight, and output variations. They also suffer from the need for substantial insulation that is both expensive and potentially hazardous to the environment (e.g oil and/or SF6 gas).
Pockels cells are known devices that have been used for measuring voltage particularly high voltage, for example see U.S. Pat. No. 5,477,134 issued to H. Hamada and U.S. Pat. No. 5,731,579 issued to G. K. Woods. A preferred form of Pockels cell for use in the present invention is an integrated optics Pockels cell such as that described in U.S. Pat. No. 5,029,273 issued Jul. 2, 1991 to Jaeger the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 5,917,316 xe2x80x9cMeasuring device for a metal-enclosed, gas-insulated high-voltage installation,xe2x80x9d issued Jun. 29, 1999, to A. Bosco, T. Hertig, and A. Kaczkowski, describes a high-voltage voltage sensor that uses a metallic enclosure providing shielding of a voltage detector located inside the enclosure against external voltage sources in order to obtain high measurement accuracy. The enclosure contains a high-voltage conductor and is filled with pressurized SF6 gas in order to sustain high-electric field stresses.
An earlier application namely U.S. Pat. No. application Ser. No. 09/204,507 filed Dec. 4, 1998 by Jaeger et al. now U.S. Pat. No. 6,289,603, the disclosure of which is incorporated herein by reference, discloses a voltage sensor similar to the present invention, but one wherein the Admittance Shielding (AS) is based primarily on dielectric shielding. This shielding while effective requires the use of materials with high permittivities which are not always easy to obtain and generally are expensive and relatively heavy.
Other systems using essentially similar voltage transducers (VTs) to those used with the present invention i.e. those that use at least one electric field sensor (EFS) to give a measurement of voltage, use admittance shielding (AS) as described in detail herein below to improve the effectiveness of the system. These known admittance shielding systems are conceived by manipulating metallic conductor (electrode) geometries (or as above indicated selecting materials with relatively large permittivities (as taught in the said Jaeger et al. application)) to attain a good level of admittance shielding.
For example one method of obtaining AS is to in effect decrease the distance between the two conductors between which voltage is to be measured. Another is to surround one conductor with the other conductor as much as possible. Both of these methods have the effect of increasing the admittance between the two conductors where the EFSs are located. Use of these methods results in the existence of high electric field stresses that must be supported with special insulation such as SF6 gas. Examples of this can be found in U.S. Pat. No. 3,938,039 xe2x80x9cVoltage measuring device for encapsulated high-voltage installations,xe2x80x9d issued Feb. 10, 1976, to W. Hermstein, G. Rosenberger, and W. Muller; U.S. Pat. No. 5,272,460 xe2x80x9cCurrent and voltage transformer for a metal-encapsulated, gas-insulated high-voltage installation,xe2x80x9d issued Dec. 21, 1993, to R. Baumgartner, K. Y. Haffler, H. Hageli, and A. Kaczkowski; U.S. Pat. No. 5,892,357 xe2x80x9cElectro-optic voltage sensor for sensing voltage in an E-field,xe2x80x9d issued Apr. 6, 1999, to G. K. Woods and T. W. Renak; and Japan Patent 05273256xe2x80x9cInsulator built-in type photo voltage sensor,xe2x80x9d issued Oct. 22, 1993, to O. Tetsuo.
The use of resistive materials in high-voltage applications is known (e.g. in a resistive divider used for dividing voltage, or at cable terminations for grading the field at the end of high voltage cables) but not for use in shielding (of EFSs) in order to measure voltage.
The present invention reduces significantly the deficiencies inherent to the existing high-voltage sensor technology.
The simple structure of the invention substantially removes the need for usually expensive and/or environmentally unfriendly insulation; may be made lighter, allowing for less expensive transportation, installation, and maintenance; and is compatible with existing standoff structures, allowing for easy construction.
When used with compact electric field sensor technology, such as the integrated optics Pockels cell, the invention offers wide bandwidth and easy interfacing with emerging digital technology.
Broadly the present invention relates to an apparatus for measuring voltage comprising an electrically isolating-section, a pair of spaced conductors between which voltage difference V is to be determined, one of said conductors being at one end of said isolating-section and another at an end of said isolating-section remote from said one end, at least one electric field sensor which senses electric field at at least one location within said isolating-section, a detector for determining a value Vd for said voltage difference V based on said electric field(s) sensed by said at least one electric field sensor, said isolating-section formed of a primarily resistive material having a resistance in the range of 25 Gxcexa9 to 50 kxcexa9 so as to provide sufficient shielding of said at least one location from sources of electric field interference of practical strength external to said apparatus so that said sources of practical strength do not change said sensed electric field sensed at said at least one location to generate a significant error in said value Vd for said voltage difference V.
Broadly the present invention also relates to an apparatus for measuring voltage comprising an electrically isolating-section, a pair of spaced conductors between which voltage difference V is to be determined, one of said conductors being at one end of said isolating-section and another at an end of said isolating-section remote from said one end, at least one electric field sensor which senses electric field at at least one location within said isolating-section, a detector for determining a value Vd for said voltage difference V based on said electric field sensed by said at least one electric field sensor, said isolating-section formed of a primarily resistive material having a resistance per unit of length in the range of 25 Gxcexa9/m to 50 kxcexa9/m so as to provide sufficient shielding of said at least one location from sources of electric field disturbance of practical strength external to said apparatus so that said sources of practical strength do not change said sensed electric field sensed at said at least one location to generate a significant error in said value Vd for said voltage difference V.
Preferably said resistance per unit of length is in the range of 1 Gxcexa9m to 500 kxcexa9/m.
Preferably said sufficient shielding reduces the error in said determined voltage Vd to less than 25% of an error in Vd that would be attained if said isolating-section was not provided.
Preferably said sufficient shielding reduces the error in said determined voltage Vd to less than 1%.
Preferably said sufficient shielding reduces the error in said determined voltage Vd to less than 0.3%.
Preferably said isolating-section is a hollow section in which the electric field sensor is mounted.
Preferably the material of the isolating-section is polyethylene doped with carbon black.
Preferably the isolating-section is enclosed in a protective sleeve protecting said isolating-section from the environment.
Preferably the electric field sensor is an optical electric field sensor.